1. Field of the Invention
The field of the invention is that of the coding of digital data belonging to a sequence of source data designed to be transmitted, or broadcast, notably in the presence of transmission noise, and of the decoding of coded data thus transmitted.
More specifically, the invention relates to a method of error correction coding that relies notably on a totally novel approach involving the concatenation of several codes, and to a decoding method that enables the making of modular decoders, with several levels of quality depending on the number of modules implemented.
The invention can be applied whenever where it is necessary to transmit digital information with a certain degree of reliability. A preferred field of application of the invention is that of digital transmission on highly noise-ridden channels. For example, the invention may be implemented for the transmission and reception of signals by satellite. It can also be used advantageously for spatial transmission towards or between spaceships and/or space probes and, more generally, whenever the reliability of the decoding is of vital importance. However, the invention can be applied similarly to any type of transmission, by r.f. or by cable.
Any digital signal, whatever may be its origin, can be coded and decoded according to the invention. It may be, for example, an image signal, a sound signal, a data signal or a multiplex of several distinct signals.
2. Description of the Prior Art
In a known way, signals such as these are generally coded by means of one or more convolutional coders. In the decoder, the original data elements are most frequently reconstructed by means of a maximum likelihood algorithm, for example a Viterbi algorithm, the decisions of which may possibly be weighted.
Convolutional codes are codes that associate at least one coded data element with each source data element, this coded data element being obtained by the summation modulo 2 of this source data element with at least one of the preceding source data elements. Thus, each coded symbol is a linear combination of the source data element to be coded and of previous data source elements taken into account. The Viterbi algorithm, taking account of a sequence of received coded symbols, gives an estimation of each data element coded at transmission, in determining the source sequence that most probably corresponds to the received sequence.
There also exists a known way of placing several coders in series, whether they are convolutional or not. In this case, the data elements coded by a first coder feed a second coder which "overcodes" these data elements. The decoding is obviously done symmetrically, in starting with the second code.
This principle, known as that of the concatenation of codes, has two types of drawbacks. Firstly, the overall efficiency rate of the coders implementing concatenated codes is low. For example, in the case of two coders in series each having a rate 1/2, the overall rate will be 1/4. If more than two decoders are used, this rate will soon become very low.
Besides, the technical making of these coders is relatively complex, notably as regards the clock signals associated with each code which have to be independent.
With respect to decoders, it has already been indicated that the algorithms generally used are maximum likelihood algorithms such as the Viterbi algorithm. These algorithms take decisions in taking account of a large number of received symbols. Clearly, the reliability of the decision increases with the number of symbols taken into account. By contrast, the higher the number, the more complex is the decoder. The memory space needed soon becomes very substantial, as do the corresponding computation times.
The integrated circuits that implement such algorithms therefore most usually rely on a compromise between cost and performance characteristics. These industrial choices do not enable the construction of decoders that correspond optimally to a given application. It is, for example, not possible to make low-cost decoders for applications where reception quality is not vital, the integrated circuits having an excessively high cost. Nor, on the other hand, are these integrated circuits adapted to the making of very high quality decoding receivers, for which the cost price is of little importance.